For cooling components of an electronic device, for example, a computer, components are often mounted on cooling bodies or provided with cooling bodies, in order to increase their surface area for dissipating heat. Typically, for supporting the cooling, fans are used that have an air stream deflected onto the cooling bodies of the temperature-critical components or elements. It is usually not necessary to operate the fans continuously at the highest rotational speed. In the case of processors, for example, only very computationally intense applications require the maximum rotational speed of the fan. Because a high rotational speed of the fan is in most cases associated with a higher noise load for the user, it is desirable to adapt the rotational speed of the fan to the conditions and to reduce the speed as much as possible. For this purpose, controllers are often used that reduce the rotational speed of the fan, for example, as a function of the temperature of the cooled components.
For driving the fans, electronically commutated fan motors that allow noise-free operation relative to collector-commutated motors and that feature higher running power and that emit less electromagnetic interference radiation are often used in electronic devices. Here, it is typical in the market to integrate a circuit unit for driving an electronically commutated fan motor with this motor together in one fan module that can be supplied with direct voltage by means of two terminal wires for operation.
FIG. 2 shows the schematic configuration of such a commercially available fan module 1 according to the state of the art. The fan module 1 includes an electronically commutated fan motor 2 that has windings or coils L1 and L2 that represent the stationary drive of the fan motor 2. For the sake of clarity, in the drawing the windings L1 and L2 are shown outside of the fan motor 2. The windings L1 and L2 are driven by a power stage 3 that has transistors T1 and T2, each connected in series with the windings L1 and L2. The power stage 3 is driven by a control stage 4 that generates, through alternating activation of the transistors T1 and T2, a rotating magnetic field of the stator.
In the fan motor 2, a rotor with a permanent magnet is moved along with this magnetic field. Fan blades are driven by the rotor. The control stage 4 typically has a magnetic sensor that detects the rotational position of the rotor, so that the power stage 3 can be driven corresponding to the detected position of the rotor. Frequently, the control stage 4 is constructed as an integrated circuit in a compact configuration including the magnetic sensor. It is further known to integrate the power stage 3 together with the control stage 4 in a common housing. As other components, the fan module 1 typically has a diode D2 arranged in series with the power stage and the control stage as polarity-inversion protection and a capacitor C2 arranged parallel to the operating voltage of the power stage and control stage for smoothing the operating voltage.
For controlling the rotational speed of the fan motor 2 of the fan module 1, a longitudinal regulating element can be provided to reduce the operating voltage of the fan module 1. The voltage drop on the longitudinal regulating element when the operating voltage of the fan module 1 decreases, however, disadvantageously leads to dissipation loss on the longitudinal regulating element and this dissipation loss is dissipated as heat.
Therefore it is advantageous to reduce the fan voltage by means of a DC converter with pulse width modulation with low losses, as is possible, for example, with the circuit shown in FIG. 3 according to the state of the art.
In the circuit according to FIG. 3, the fan module 1 of FIG. 2 is supplied with current by means of a DC converter, wherein the DC converter has a switching transistor T3, a coil L3, a recovery diode D3, occasionally also referred to as free-wheeling diode D3, and a capacitor C3. The switching transistor T3 is driven with a rectangular, pulse width modulated signal PWM through which the switching transistor T3 is either completely turned on or completely turned off, by means of which the dissipation loss in the switching transistor T3 is minimized. The pulse duty ratio of the pulse-width modulation determines the output voltage of the DC converter and thus also the rotational speed of the fan motor 2.
However, one disadvantage in this type of rotational speed control of the fan motor 2 of a commercially available fan module 1 is that it requires, in addition to the switching transistor T3 and the recovery diode D3, the use of the DC converter made from the coil L3 and the capacitor C3 which, first, increases the cost of the drive circuit and which, second, is associated with additional space requirements, for example, on a main circuit board in a computer.
As another possibility for the rotational speed control of a fan motor, it is known to use the transistors of a power stage of a fan module as switching elements for pulse width modulation. In this case, the control stage of the fan module is designed for a corresponding driving of the power stage and the fan module has, in addition to terminals for providing the operating voltage, an input on which the signal is applied for pulse width modulation. Such a fan module, however, is more expensive and used less universally due to the more complicated internal wiring, due to the additional terminal wire, and due to the terminal plug with several terminals.